1. Field of Invention
The present invention relates to maturated unsaturated polyester molding compositions or compounds, wherein the unsaturated polyester resin has both hydroxyl and carboxyl terminal groups present, comprising a dual thickening system of metallic oxides and hydroxides of calcium or magnesium and a polyisocyanate, the viscosity index, i.e., decrease in viscosity with increase in temperature, of which is reduced when compared to the same polyester matrix thickened with calcium or magnesium oxide or hydroxide alone, and which possess great advantage when compared to the same polyester matrix thickened with polyisocyanate alone. In fact, the desirable properties such as viscosity index, handleability, and slow initial viscosity rise attained in any particular polyester matrix of the invention is not achievable, so far as is known, with the same unsaturated polyester by using either metal oxide or hydroxide or polyisocyanate alone. Hereinafter polyisocyanate will refer to di, tri, or higher functionality isocyanates.
2. Prior Art
The rapid growth of molding compounds made from unsaturated polyester resins has been due in large part to the development of thickenable systems using metallic oxides or hydroxides. This development, first disclosed by Fisk in U.S. Pat. No. 2,628,209, consisted of adding a metallic oxide or hydroxide to an unsaturated polyester resin. Amounts of such metallic oxide or hydroxide thickening agents of between one hundred and two hundred percent, based upon the stoichiometric amount required for complete reaction with all carboxyl groups of the unsaturated polyester, are today conventional. This enabled the compounder to prepare bulk molding compounds (BMC) the initial viscosity of which was low, in order to minimize glass degradation in the double arm mixer during glass addition. After addition of the reinforcing agent, primarily chopped glass, the viscosity rise occurs on maturation, mainly 24 to 72 hours or longer; and at that point the molding compound had, at room temperature, a firm doughy consistency, was handleable without wetting the hands of the operator, and had sufficient viscosity to push the glass throughout the mold. The development of such thickening systems also permitted the development of sheet molding compounds (SMC), in which a filled or unfilled resin system with viscosities in the range of 600 to 50,000 centipoise could impregnate either a glass mat or the chopped glass in mat form and form a sheet consisting of intermixed resin matrix and glass. This material then could be subsequently maturated either at room temperatures or slightly higher temperatures to achieve viscosities at room temperature of the SMC matrix of somewhere between ten to one hundred million centipoise. The material in this form was handleable, could be slit, cut, rolled, or formed and could be added to the mold and had sufficient viscosity to push the glass ahead of it with the resin matrix.
However, BMC molding compounds made without the use of thickening agents, but using high surface fillers (such as asbestos) which greatly increase the viscosity of the resin, exhibit different flow characteristics than compounds whose initial viscosity is low and which are thickened through the use of metallic oxide or hydroxide. The difference in the flow of these two types of compounds has never readily been understood. An examination of the viscosity at elevated temperatures of molding compounds built with high clay loadings or clay carbonate-asbestos loadings, versus compounds made with carbonates which subsequently have been thickened with metallic oxides or hydroxides, indicates that compounds thickened thusly undergo radical viscosity decreases with rising temperature, whereas compounds which are thickened primarily through the use of high surface fillers exhibit a much lower decrease in viscosity.
The manufacturer of SMC molding compositions requires low initial viscosity of the SMC matrix to adequately wet out the glass; in the making of high quality BMC compounds, low viscosities are also desired in the mixer during glass addition. Therefore, the viscosity should not rise too rapidly initially, but high viscosity is required prior to molding to distribute the glass uniformly throughout the part being molded. The interest is not only in high viscosity at room temperature, but minimum decrease in viscosity at molding temperatures.
It quickly became apparent that Mg(OH).sub.2, MgO, CaO and Ca(OH).sub.2 -thickened systems all possessed a similar temperature dependence. For example, a compound which had an initial maturated viscosity at room temperature of fifty million centipoise could drop down to viscosities on the order of two hundred thousand to four hundred thousand centipoise at molding temperatures, indicating an over one hundred-fold decrease in viscosity. Other methods of thickening polyester systems were accordingly investigated, including the use of polyisocyanate as sole thickening agent. However, the use of polyisocyanate as sole thickening agent is not satisfactory for the following reasons:
A. The isocyanate reacts rapidly with terminal hydroxylgroups, but slowly with carboxyl groups with evolution of CO.sub.2. Most commercial unsaturated polyester resins have mixed terminal groups of both hydroxyl and carboxyl. Entrapment of gas formed by reaction of carboxyl groups and isocyanate results, with production of cheesy molding compounds that are difficult to handle, and produces undesirable surface properties in the ultimate molded product.
B. Unsaturated polyester resins with only hydroxyl terminal groups, or primarily hydroxyl terminal groups (as taught by McGranaghan, U.S. Pat. No. 3,824,201) are expensive to make, not generally available, and require large quantities of expensive polyisocyanate to achieve a tack-free handleable matrix. Initial viscosity rise is extremely rapid, causing glass wetout problems.
C. Unsaturated polyester resins with roughly equivalent hydroxyl to carboxyl ratios cannot be thickened solely with polyisocyanates to produce desirable molding compounds. At high polyisocyanate levels, reaction rates are high and sufficient CO.sub.2 is liberated to form a crumbly matrix. At levels of polyisocyanate necessary to react only with the hydroxyl terminal groups, a tacky, soft, non-handleable matrix is obtained.